A conventional turbo-tray dryer as shown in FIG. 10 is a continuous dryer consisting of a stack of rotating annular shelves S in the center of which turbo-type fans T revolve to circulate the air over the shelves. Wet material F enters through the roof, falling onto the top shelf as it rotates beneath the feed opening. After completing one revolution, the material is wiped by a stationary wiper W through radial slots on an upper shelf to a lower shelf where it is spread into a uniform pile by a stationary leveler L. The action is repeated on each shelf, with transfers occurring once each revolution. From the last shelf, material is discharged through the bottom of the dryer D and the air A is exhausted through an upper stack. However, such a turbo-tray dryer still has the following defects:
1. In order to wipe the drying material on an upper shelf to a lower shelf for its uniform drying, a plurality of wipers and levelers must be provided to increase their installation cost and maintenance problems.
2. As shown in FIG. 10, the hot air stream, flowing radially over each shelf as driven by the turbo fans as guided by the plural annular shelves stacked vertically, will reciprocatively impact the housing wall and the shelves to cause turbulent flow and energy loss, thereby reducing the dryer efficiency.
3. The dried product as wiped onto the lower shelves will become light to be floated and laden on rising air stream to possibly clog the suction ports or blades of the fans.
The present inventor has found the defects of such a conventional turbo-tray dryer and invented the present dryer exerting a spirally floating air stream.